The invention relates to a device for flushing the optical path of a UV laser beam, having a casing separating the optical path from the external atmosphere.
It is known in the prior art to flush the optimal path of a UV laser beam (that is to say of a laser beam having a wavelength in the ultraviolet range) with nitrogen (J. A. R. Samson (Ed.): Techniques of vacuum ultraviolet spectroscopy, New York, 1967). This prior art is based on recognition of the fact that UV laser beams, in particular with a wavelength of 193 nm, produce ozone in atmospheric oxygen, and this causes a range of technical problems. In particular, ozone should be avoided because it can irritate the airways. Furthermore, however, ozone also absorbs the laser radiation and thus attenuates the laser beam. In order to avoid this, in the prior art, the entire beam path from the output-coupling optical unit of the laser system to the object to be illuminated is encapsulated in such a way that the encapsulated space is hermetically sealed from the space outside. The encapsulation involves a casing around this space with a suitable wall. Next, according to the prior art, nitrogen is introduced into the space with a slightly positive pressure relative to the atmosphere. Once the entire system has been filled with nitrogen, the laser radiation can reach the workpiece without losses and without producing detrimental ozone. A further possible way which the prior art mentioned above proposes for avoiding the formation of ozone is to evacuate the space containing the optical path. However, this elaborate measure is used primarily for extremely short UV wavelengths, that is to say wavelengths considerably shorter than 193 nm, since in cases such as these the radiation is absorbed in the gas.
The present invention is based on recognition of the fact that, in the case of UV laser radiation, flushing the optical path with nitrogen may be detrimental to optical components in the optical path of the laser, for example windows, lenses, mirrors, prisms, etc., and in particular the optical quality of the surfaces of the optical components may be impaired. This is in particular true of optical components made from CaF2, for which surface flaking may result. Optical components of this type in the optical path of a laser beam are, in particular, needed for focusing and/or rendering uniform the laser beam on the workpiece to be illuminated.
The present invention is furthermore based on recognition of the surprising fact that the aforementioned technical problems which occur when nitrogen is used no longer occur if the optical path of the UV laser beam is flushed with noble gas. Argon, in particular, has been found to be highly suitable for this purpose.
The solution according to the invention exhibits particularly good results for excimer laser radiation with a wavelength of 193 nm and for the fifth harmonic of Nd:YAG lasers (212.8 nm).
The invention is thus suitable, in particular, for wavelengths of less than 220 nm down to shorter wavelengths, so long as the radiation is not absorbed by the noble gas. This leads to a preferred wavelength range of from about 220 nm to about 150 nm.